The Commerce Commission is seeking more information from farmers and others who entered into interest rate swaps, with the aim of protecting themselves from rising interest rates.
Some farmers who took up the option say they've suffered significant financial losses as a result.
The commission is investigating whether banks misled clients in the way they marketed the swaps.
It says it hasn't decided yet whether the Fair Trading Act has been breached, but is concerned enough to probe further.
It has information from about 60 complainants so far, but says it needs to hear from more people, including those who were happy with the arrangement, to get the full picture.